JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 8, No. 3, June 2006, p. 1243 - 1246
High-resolution periodical structure on polycarbonate
using holographic interferometry and electroforming
process
WEN-CHUNG CHANGa,*, WEI-CHING CHUANGb, CHI-TING HOc, KAO-FENG YARNd
a
Department of Electronic Engineering, Southern Taiwan University of Technology, Tainan 710, Taiwan, ROC
b
Department of Electro-Optics Engineering, National Formosa University, Huwei, Yulin, Taiwan, ROC
c
Department of Mechanical Design Engineering, National Formosa University, Huwei, Yulin, Taiwan, ROC
d
Department of Electronic Engineering, Far East College, Tainan 744, Taiwan, ROC
A procedure for prototyping a periodic structure on polycarbonate at submicron order using holographic interferometry and
electroplating metal molding processes is demonstrated. The holographic interference using a He-Cd (325nm) laser was first
used to create periodic line structure on an i-line sub-micron positive photoresist film. Pattern was then transferred to a metal
mold using Nickel-Cobalt electroforming. Final line pattern on a polycarbonate thin film was formed from the metal mold
using a hot press machine. The technique allows for accurate control for the transfer of a grating period and depth. The
grating pattern on the polycarbonate polymer produced by the metal mold gives an average error of less than 1.7% in the
grating period and an average error of 5.7% for depth reproduction.
(Received May 5, 2006; accepted May 18, 2006)
Keywords: Polycarbonate, Holographic interferometry, Submicron grating
1. Introduction
Gratings are widely used in the integrated photonic
devices for purposes such as spectral filtering,
multiplexing, routing, fan-out, focusing, collimating, etc.
[1-6]. Among all the material used, polymer gratings are of
great interest for their low cost and easy fabrication.
Recently, demands of low cost and high-resolution
lithographic technique have attracted a considerable
interest in the pursuit of new ways to create a more
efficient grating structure. Typical techniques for
patterning gratings on polymers include holographic
surface relief grating [7-9], electro-beam (e-beam)
lithography [10], laser-beam direct writing [11], X-ray
mask technology [12], and phase mask lithography [13-14].
Although these tools provide high precision in
manufacturing, it often limited by its size of the device
which it can actually be manufactured. These mentioned
tools are also hard to get and relatively expensive. On the
other hand, the unique capacity of holographic lithography
for high-resolution and large-area pattern transfer offers
significant advantages over the other techniques. The
holographic
interferometrical
technique
provides
flexibility in the grating period and depth by changing the
incident beam angles and the irradiation time. In addition,
the theoretical limit of the frequency of the interference
pattern produced by two intersecting beams is half of the
wavelength of the incident beam. Thus, the grating period
is only limited by the wavelength of the light source.
The advanced technology of LIGA-like process
eliminates the problem of tool wear and material
machine-ability, and can achieve sub-micro accuracy.
There are many replication processes that are simple and
involve fairly easy fabrication; such as hot-embossing [15],
UV-embossing [16], and micro-transfer molding method
[17,18]. If these molding processes are combined with a
LIGA-like process that can produce a mold for the
subsequent molding processes, it may have great potential
for mass production [19]. Converting the plastic mold into
the metallic mold is the basic concept of a LiGA-like
process. There are two issues of electroforming to make
micro-molds with enough thickness (at least 2 to 3 µm) for
practical molding process; one is the deformation of
electroforming molds after a length time, the other is the
low growth rate in electroforming itself. Even thought
some additives are used to control the deformation induced
by internal stress in electroforming process, real time
monitoring the use of this additives is no sophisticated, the
deformation of micro-molds after electroforming is still
unpredictable [20]. In here, we have used two steps to
improve the problems. First step is to control the
electroforming thickness under 300 µm with less internal
stress and no deformation. The second step is to apply a
mold plate enough thick for supporting molding pressure.
By
using
the
holographic
interferometry,
electroforming and stamping process, this work
demonstrates a new fabricating technique of period
structure on polycarbonate thin film. Based on the
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Wen-Chung Chang, Wei-Ching Chuang, Chi-Ting Ho, Kao-Feng Yarn
preliminary experimental results, it is found that it is
possible to replicate grating period and depth with high
precision. The metal mold also has the ability to show
more reliable and reproducible results.
2. Methodology and results
The technique of forming grating patterns on the
polycarbonate thin film involved three processing steps:
First, a grating pattern is holographically exposed using
two-beam interference pattern on a positive photoresist
film. This produces a master that can be subsequently
transferred to a Nickel-Cobalt (Ni-Co) metal mold. The
metal mold then is used to transfer the final gratings
pattern onto a polycarbonate using a hot press process.
2.1 Holographic grating
The master grating patterns on a positive photoresist
(Ultra123, Microchem. Corp. MA) were holographically
exposed using a two-beam interferometer technique [21].
Based on the results, we found that the grating period and
the corresponding depth of the grating pattern can be
accurately controlled down to an error rate of less than 1%.
We also found that a high aspect ratio of almost 1:1
between the depth and the period of the grating structure
could be obtained using this process. The profiles of the
grating were measured by atomic force microscope (AFM)
and shown in Fig. 1.
Fig. 1. The AFM picture and measurement result for the
grating on Ultra123 positive photoresist (505 nm grating
period and 333 nm grating depth).
2.2 Molding process
Electroplating process using nickel-cobalt alloy was
applied to create the metal mold. To increase the adhesion,
a 200 nm nickel thin film first sputtered onto the positive
photoresist mold to serve as a seed layer for the
subsequent electroplating of Ni-Co alloy. The sample is
then immersed into a tank filled with solution of nickel
and cobalt salt diluted in boric acid. Bars of nickel and
cobalt hang in the tank served as anodes to keep the
solution in balance. An electrical current passed through
the solution, caused atoms from each element to form a
metal on the being plated. For our setting, Boric acid:
30 – 40 g/l. and Co concentration between 1 to 20 g/l. and
70 g/l were used. The pH level was kept around 4.0 and
temperature was maintained near 55 oC, and the solution
was agitated by a Magnetic stirrer (see Table 1).
Table 1. Ni-Co alloy electrolyte composition.
Ni concentration
Co concentration
Boric acid
Current density
PH
Temperature
Agitation
70 g/L
1~20% (w/o) in sol.
30~40 g/L
1~10 ASD (Adm-2)
4.0 ± 0.5
55 ± 1ºC
Magnetic stirrer
In order to improve the thickness uniformity of
electroplating mold, which is usually in concave shape, a
secondary cathode was applied. The secondary cathode
was placed at a specific distance (less than 2.5 mm) from
the primary cathode during the electroplating process. Two
power supplies are used, one is for the primary cathode,
and the other one is for the secondary electrode. Both
cathodes share with the same anodes, and the applied
current density is the same for both cathodes. The purpose
of the second cathode was to reduce the local ion
concentration of the double layer surface plating area. This
process hindered the grow rate on the edges of the
deposited mold. When high deposition rate occurs on the
edges of the plating field, the secondary cathode becomes
a frame shaped like the plating area, which adapts to
locally reduce the deposition rate on the primary cathode.
After 15 hours of continuous deposition at a
deposition rate of 15.6 µm/hour, a 234 µm thick layer of
Ni-Co metal is deposited above the surface of the positive
resist mold. Basically, the mold was formed, but it still not
thick enough for subsequent molding process. A thick
nickel plate was clamped onto the metal mold surface for
continuous electroplating. After 30 hours, the thick plate
bonded onto the Nickel mold and formed into a piece. This
enables a 5 mm thick metal mold to be formed. The
positive photoresist was then stripped away using acetone.
The metallization process is depicted in Fig. 2. Picture
taken from the scanning electron microscope indicated that
the grating pattern structure of the metal mold was well
transferred (see Fig. 3). Based on the measurement taken
from atomic force microscope (AFM) and SEM, the
gratings on the metal mold transferred from a master
grating were slightly off from the original dimension. For
example, the grating period was off by 2 nm and the depth
was reduced by 26 nm for a profile of 505 nm period and
333 nm depth, while the grating period was off by 2 nm
and the depth was reduced by 34 nm for a profile of
703 nm period and 399 nm depth.
High-resolution periodical structure on polycarbonate using holographic interferometry and electroforming process
1245
formation at different pressures. However, the temperature
appeared to be critical in the process. Our result indicated
that the grating pattern could not be formed on the
polymer when the temperature was less than 130 oC. When
the temperature was over 145 oC, tiny bubbles started to
develop in the samples. The out-gassing caused surface
damage on the sample. Therefore, the idea operating
temperature was found to be between 135 oC to 140 oC.
The thickness of the PC sample was about 2 mm. The
profiles of the gratings on PC were measured using an
AFM. Based on the measurement, when the depths of the
gratings were less than 370 nm, grating patterns
transferred quite accurately from the metal mold.
Comparing the two SEM micrographs of PC and Nickel
metal mold (grating period of 494 nm) indicated that the
patterns were successful (Fig. 4(a) and (b)). The AFM
micrograph result shows that the gratings on a PC
substrate transferred from a metal grating, a profile of
503 nm period and 307 nm depth, are slightly off from the
metal mold dimension (Fig. 4(b)). The grating period was
off by 9 nm while the depth was reduced by 15 nm. Based
on the results from three different metal molds, which are
701 nm, 610 nm, and 503 nm periods with 365 nm, 343
nm, and 307 nm depth respectively, the reduction shows
an average of 1.7% or 10 nm average reductions in the
periods. The depths on the other hand were reduced by as
much as 5.7% or 20 nm on average.
Fig. 2. Electroplating process for Ni-Co grating mold.
(a)
Fig. 3. The SEM picture of gratings on Ni-Co mold (503
nm grating period and 307 nm grating depth).
The patterned metal mold was used as a master mold
to transfer the grating pattern onto a polycarbonate (PC)
material thin film (supplied by Hsintou Company, Taiwan)
using typical thermal pressing technique. The PC is a
suitable material for optical devices due to its good
physical and optical properties. A process similar to
injection molding was used. Metal mold was placed on a
stainless substrate in a hot pressing machine. Two
processing parameters including working temperature and
pressure were investigated. One test we conducted was
adjusting pressure in the fabrication process between 5 to
20 kg/cm 2 with a holding time of 180 seconds. However,
no significant difference was found in the gratings
(b)
Fig. 4. The SEM and AFM micrographs of gratings on
PC polymer (a) SEM (b) AFM. (494 nm grating period
and 292 nm grating depth).
In order to investigate the performance of the
diffraction gratings, the first order diffraction efficiencies,
1246
Wen-Chung Chang, Wei-Ching Chuang, Chi-Ting Ho, Kao-Feng Yarn
η, of the gratings with period of 494 nm and depth 292 nm
printed on the PC polymer were measured by the He-Cd
laser at a wavelength of 325 nm. The first order diffraction
efficiency of 18.51% was attained during the irradiation.
3. Conclusion
In conclusion, we have successfully created a process
for rapid production of submicron range gratings by using
both electroplating-molding and holographic interference
techniques. A large aspect ratio on the grating pattern
could be obtained, and reproduction of the grating on a
polycarbonate polymer could be achieved. The grating
period and depth on the polymer gratings exhibit small
difference from the original designed grating pattern. This
process shows great potential for mass production of any
period of grating structure.
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*
Corresponding author: [email protected]